As much as I like my 115 Merc, I'd have to say the 115 Johnson of the same era is a better motor. Turns higher RPM, is smoother, has a lower profile that looks better. Both motors are tried and true, reliable designs.
I give the nod to the Merc on corporate stability, but for a popular (including parts issues), out-of-warranty motor, I don't think that counts for very much.